Frequency-Dependent Attenuation and Velocity Characteristics of Magnetically Lossy Materials

In many situations, the magnetic properties of sub-surface materials are often considered unimportant when compared to their ´dielectric´ characteristics (i.e., permittivity and conductivity). However, if significant amounts of magnetic minerals exist, such as magnetite, hematite, maghemite and/or iron in its free state, then the relaxation phenomena of these magnetically lossy particles can have an overriding effect on the complex effective permittivity spectrum of the material. In this paper, the effective permittivity, attenuation and propagation characteristics of a range of nano-to-micro scale quartz/magnetite mixtures are investigated with the aim of determining how lossy magnetic minerals affect the macroscopic properties of the material as a whole. In addition, the measured results are compared to popular ´dielectric-based´ mixing models (such as the Complex Refractive Index Model CRIM) and the nature of the magnetic relaxation mechanisms is discussed from the aspect of composite mediums. Results indicate that even relatively small amounts of magnetite can have a considerable effect on both signal attenuation and wave propagation velocity and that the current range of mixing models are inadequate for the description of magnetically lossy mixtures.